Polycarbonate has superior mechanical properties such as impact resistance, high heat resistance and transparency, and is widely used for applications such as various types of machine parts, optical discs, automobile parts, etc.
In the past, the above mentioned polycarbonate has been produced using a method consisting of a direct reaction of an aromatic dihydroxy compound such as bisphenol and phosgene (interfacial method), or a method consisting of a transesterification reaction of an aromatic dihydroxy compound and a dicarbonate (hot-melt method).
Among those described above, the hot-melt method (hot-melt polycondensation) has an advantage over the interfacial method since it is possible to produce a polycarbonate at a relatively low cost; furthermore, toxic materials such as phosgene are not used; therefore, it is desirable from the environmental standpoint as well.
Incidentally, in conventional polycarbonates, the stability becomes inferior in some cases at the time of hot-melt processes such as molding, and a part of the material decomposes; as a result, problems such as a decrease in the molecular weight and discoloration occur at times.
As a method to improve the stability of the polycarbonate, in the past, a method consisting of adding an additive suitable for the application purpose after performing a polycondensation for an aromatic dihydroxy compound, and a dicarbonate in the presence of an alkaline catalyst compound while the polycarbonate reaction product is still in a molten state in the hot-melt polycondensation method, and kneading the material to reduce the occurrence of the heat history that the polycarbonate is subjected to during manufacturing (Japanese Kokai Patent Application No. Hei 4 1992!-103626), and a method consisting of adding an acid compound after performing a hot-melt polycondensation of an aromatic dihydroxy compound and a dicarbonate in the presence of an alkaline catalyst compound (Japanese Kokai Patent Application No. Hei 4 1992!-175368, and Japanese Kokai Patent Application No. Hei 4 1992!-328124) are disclosed.
As a result of continued research into manufacturing methods of polycarbonates using the hot-melt polycondensation method, the inventors discovered that a polycarbonate composition with further improved stability, for example heat stability and color stability at the time of molding and high weather resistance and moisture resistance, can be produced when a sulfur-containing compound with a pKa value less than 3 or derivatives thereof, and a specific amount of water is added and mixed after performing a hot-melt polycondensation reaction for an aromatic dihydroxy compound and a dicarbonate and while the reaction product polycarbonate is still in a molten state, and accomplished the present invention.
Also, in the interfacial polymerization method, a method consisting of adding water in an amount of 0.2-20 parts by weight for 100 parts by weight of the polycarbonate at the time of kneading of the polycarbonate powder produced, and extruding while degassing is being done so as to reduce volatile impurities such as methylene chloride has been disclosed (Japanese Kokoku Patent No. Hei 5 1993!-27647).